src/Pure/Isar/method.ML
author wenzelm
Sun Mar 01 23:36:12 2009 +0100 (2009-03-01)
changeset 30190 479806475f3c
parent 30165 6ee87f67d9cd
child 30234 7dd251bce291
permissions -rw-r--r--
use long names for old-style fold combinators;
     1 (*  Title:      Pure/Isar/method.ML
     2     Author:     Markus Wenzel, TU Muenchen
     3 
     4 Isar proof methods.
     5 *)
     6 
     7 signature BASIC_METHOD =
     8 sig
     9   val FINDGOAL: (int -> thm -> 'a Seq.seq) -> thm -> 'a Seq.seq
    10   val HEADGOAL: (int -> thm -> 'a Seq.seq) -> thm -> 'a Seq.seq
    11   type method
    12   val trace_rules: bool ref
    13 end;
    14 
    15 signature METHOD =
    16 sig
    17   include BASIC_METHOD
    18   val apply: Position.T -> (Proof.context -> method) -> Proof.context -> thm list -> cases_tactic
    19   val RAW_METHOD_CASES: (thm list -> cases_tactic) -> method
    20   val RAW_METHOD: (thm list -> tactic) -> method
    21   val METHOD_CASES: (thm list -> cases_tactic) -> method
    22   val METHOD: (thm list -> tactic) -> method
    23   val fail: method
    24   val succeed: method
    25   val insert_tac: thm list -> int -> tactic
    26   val insert: thm list -> method
    27   val insert_facts: method
    28   val SIMPLE_METHOD: tactic -> method
    29   val SIMPLE_METHOD': (int -> tactic) -> method
    30   val SIMPLE_METHOD'': ((int -> tactic) -> tactic) -> (int -> tactic) -> method
    31   val defer: int option -> method
    32   val prefer: int -> method
    33   val cheating: bool -> Proof.context -> method
    34   val intro: thm list -> method
    35   val elim: thm list -> method
    36   val unfold: thm list -> Proof.context -> method
    37   val fold: thm list -> Proof.context -> method
    38   val atomize: bool -> method
    39   val this: method
    40   val fact: thm list -> Proof.context -> method
    41   val assumption_tac: Proof.context -> int -> tactic
    42   val assumption: Proof.context -> method
    43   val close: bool -> Proof.context -> method
    44   val trace: Proof.context -> thm list -> unit
    45   val rule_tac: thm list -> thm list -> int -> tactic
    46   val some_rule_tac: thm list -> Proof.context -> thm list -> int -> tactic
    47   val intros_tac: thm list -> thm list -> tactic
    48   val rule: thm list -> method
    49   val erule: int -> thm list -> method
    50   val drule: int -> thm list -> method
    51   val frule: int -> thm list -> method
    52   val set_tactic: (thm list -> tactic) -> Proof.context -> Proof.context
    53   val tactic: string * Position.T -> Proof.context -> method
    54   val raw_tactic: string * Position.T -> Proof.context -> method
    55   type src = Args.src
    56   datatype text =
    57     Basic of (Proof.context -> method) * Position.T |
    58     Source of src |
    59     Source_i of src |
    60     Then of text list |
    61     Orelse of text list |
    62     Try of text |
    63     Repeat1 of text |
    64     SelectGoals of int * text
    65   val primitive_text: (thm -> thm) -> text
    66   val succeed_text: text
    67   val default_text: text
    68   val this_text: text
    69   val done_text: text
    70   val sorry_text: bool -> text
    71   val finish_text: text option * bool -> Position.T -> text
    72   val print_methods: theory -> unit
    73   val intern: theory -> xstring -> string
    74   val defined: theory -> string -> bool
    75   val method: theory -> src -> Proof.context -> method
    76   val method_i: theory -> src -> Proof.context -> method
    77   val add_methods: (bstring * (src -> Proof.context -> method) * string) list
    78     -> theory -> theory
    79   val add_method: bstring * (src -> Proof.context -> method) * string
    80     -> theory -> theory
    81   val method_setup: bstring -> string * Position.T -> string -> theory -> theory
    82   val syntax: (Context.generic * Args.T list -> 'a * (Context.generic * Args.T list))
    83     -> src -> Proof.context -> 'a * Proof.context
    84   val simple_args: (Args.T list -> 'a * Args.T list)
    85     -> ('a -> Proof.context -> method) -> src -> Proof.context -> method
    86   val ctxt_args: (Proof.context -> method) -> src -> Proof.context -> method
    87   val no_args: method -> src -> Proof.context -> method
    88   type modifier
    89   val sectioned_args: (Context.generic * Args.T list -> 'a * (Context.generic * Args.T list)) ->
    90     (Args.T list -> modifier * Args.T list) list ->
    91     ('a -> Proof.context -> 'b) -> src -> Proof.context -> 'b
    92   val bang_sectioned_args:
    93     (Args.T list -> modifier * Args.T list) list ->
    94     (thm list -> Proof.context -> 'a) -> src -> Proof.context -> 'a
    95   val bang_sectioned_args':
    96     (Args.T list -> modifier * Args.T list) list ->
    97     (Context.generic * Args.T list -> 'a * (Context.generic * Args.T list)) ->
    98     ('a -> thm list -> Proof.context -> 'b) -> src -> Proof.context -> 'b
    99   val only_sectioned_args:
   100     (Args.T list -> modifier * Args.T list) list ->
   101     (Proof.context -> 'a) -> src -> Proof.context -> 'a
   102   val thms_ctxt_args: (thm list -> Proof.context -> 'a) -> src ->
   103     Proof.context -> 'a
   104   val thms_args: (thm list -> 'a) -> src -> Proof.context -> 'a
   105   val thm_args: (thm -> 'a) -> src -> Proof.context -> 'a
   106   val goal_args: (Args.T list -> 'a * Args.T list) -> ('a -> int -> tactic)
   107     -> src -> Proof.context -> method
   108   val goal_args': (Context.generic * Args.T list -> 'a * (Context.generic * Args.T list))
   109     -> ('a -> int -> tactic) -> src -> Proof.context -> method
   110   val goal_args_ctxt: (Args.T list -> 'a * Args.T list) ->
   111     (Proof.context -> 'a -> int -> tactic) -> src -> Proof.context -> method
   112   val goal_args_ctxt': (Context.generic * Args.T list -> 'a * (Context.generic * Args.T list)) ->
   113     (Proof.context -> 'a -> int -> tactic) -> src -> Proof.context -> method
   114   val parse: OuterLex.token list -> text * OuterLex.token list
   115 end;
   116 
   117 structure Method: METHOD =
   118 struct
   119 
   120 (** generic tools **)
   121 
   122 (* goal addressing *)
   123 
   124 fun FINDGOAL tac st =
   125   let fun find i n = if i > n then Seq.fail else Seq.APPEND (tac i, find (i + 1) n)
   126   in find 1 (Thm.nprems_of st) st end;
   127 
   128 fun HEADGOAL tac = tac 1;
   129 
   130 
   131 
   132 (** proof methods **)
   133 
   134 (* datatype method *)
   135 
   136 datatype method = Meth of thm list -> cases_tactic;
   137 
   138 fun apply pos meth_fun ctxt facts goal = Position.setmp_thread_data_seq pos
   139   (fn () => let val Meth meth = meth_fun ctxt in meth facts goal end) ();
   140 
   141 val RAW_METHOD_CASES = Meth;
   142 
   143 fun RAW_METHOD tac = RAW_METHOD_CASES (NO_CASES o tac);
   144 
   145 fun METHOD_CASES tac = RAW_METHOD_CASES (fn facts =>
   146   Seq.THEN (ALLGOALS Goal.conjunction_tac, tac facts));
   147 
   148 fun METHOD tac = RAW_METHOD (fn facts => ALLGOALS Goal.conjunction_tac THEN tac facts);
   149 
   150 val fail = METHOD (K no_tac);
   151 val succeed = METHOD (K all_tac);
   152 
   153 
   154 (* insert facts *)
   155 
   156 local
   157 
   158 fun cut_rule_tac rule =
   159   Tactic.rtac (Drule.forall_intr_vars rule COMP_INCR revcut_rl);
   160 
   161 in
   162 
   163 fun insert_tac [] i = all_tac
   164   | insert_tac facts i = EVERY (map (fn th => cut_rule_tac th i) facts);
   165 
   166 val insert_facts = METHOD (ALLGOALS o insert_tac);
   167 fun insert thms = METHOD (fn _ => ALLGOALS (insert_tac thms));
   168 
   169 fun SIMPLE_METHOD tac = METHOD (fn facts => ALLGOALS (insert_tac facts) THEN tac);
   170 fun SIMPLE_METHOD'' quant tac = METHOD (fn facts => quant (insert_tac facts THEN' tac));
   171 val SIMPLE_METHOD' = SIMPLE_METHOD'' HEADGOAL;
   172 
   173 end;
   174 
   175 
   176 (* shuffle subgoals *)
   177 
   178 fun prefer i = METHOD (K (Tactic.defer_tac i THEN PRIMITIVE (Thm.permute_prems 0 ~1)));
   179 fun defer opt_i = METHOD (K (Tactic.defer_tac (the_default 1 opt_i)));
   180 
   181 
   182 (* cheating *)
   183 
   184 fun cheating int ctxt = METHOD (K (setmp quick_and_dirty (int orelse ! quick_and_dirty)
   185     (SkipProof.cheat_tac (ProofContext.theory_of ctxt))));
   186 
   187 
   188 (* unfold intro/elim rules *)
   189 
   190 fun intro ths = SIMPLE_METHOD' (CHANGED_PROP o REPEAT_ALL_NEW (Tactic.match_tac ths));
   191 fun elim ths = SIMPLE_METHOD' (CHANGED_PROP o REPEAT_ALL_NEW (Tactic.ematch_tac ths));
   192 
   193 
   194 (* unfold/fold definitions *)
   195 
   196 fun unfold_meth ths ctxt = SIMPLE_METHOD (CHANGED_PROP (LocalDefs.unfold_tac ctxt ths));
   197 fun fold_meth ths ctxt = SIMPLE_METHOD (CHANGED_PROP (LocalDefs.fold_tac ctxt ths));
   198 
   199 
   200 (* atomize rule statements *)
   201 
   202 fun atomize false = SIMPLE_METHOD' (CHANGED_PROP o ObjectLogic.atomize_prems_tac)
   203   | atomize true = RAW_METHOD (K (HEADGOAL (CHANGED_PROP o ObjectLogic.full_atomize_tac)));
   204 
   205 
   206 (* this -- resolve facts directly *)
   207 
   208 val this = METHOD (EVERY o map (HEADGOAL o Tactic.rtac));
   209 
   210 
   211 (* fact -- composition by facts from context *)
   212 
   213 fun fact [] ctxt = SIMPLE_METHOD' (ProofContext.some_fact_tac ctxt)
   214   | fact rules _ = SIMPLE_METHOD' (ProofContext.fact_tac rules);
   215 
   216 
   217 (* assumption *)
   218 
   219 local
   220 
   221 fun cond_rtac cond rule = SUBGOAL (fn (prop, i) =>
   222   if cond (Logic.strip_assums_concl prop)
   223   then Tactic.rtac rule i else no_tac);
   224 
   225 in
   226 
   227 fun assumption_tac ctxt =
   228   assume_tac APPEND'
   229   Goal.assume_rule_tac ctxt APPEND'
   230   cond_rtac (can Logic.dest_equals) Drule.reflexive_thm APPEND'
   231   cond_rtac (can Logic.dest_term) Drule.termI;
   232 
   233 fun assumption ctxt = METHOD (HEADGOAL o
   234   (fn [] => assumption_tac ctxt
   235     | [fact] => solve_tac [fact]
   236     | _ => K no_tac));
   237 
   238 fun close immed ctxt = METHOD (K
   239   (FILTER Thm.no_prems
   240     ((if immed then ALLGOALS (assumption_tac ctxt) else all_tac) THEN flexflex_tac)));
   241 
   242 end;
   243 
   244 
   245 (* rule etc. -- single-step refinements *)
   246 
   247 val trace_rules = ref false;
   248 
   249 fun trace ctxt rules =
   250   if ! trace_rules andalso not (null rules) then
   251     Pretty.big_list "rules:" (map (ProofContext.pretty_thm ctxt) rules)
   252     |> Pretty.string_of |> tracing
   253   else ();
   254 
   255 local
   256 
   257 fun gen_rule_tac tac rules facts =
   258   (fn i => fn st =>
   259     if null facts then tac rules i st
   260     else Seq.maps (fn rule => (tac o single) rule i st) (Drule.multi_resolves facts rules))
   261   THEN_ALL_NEW Goal.norm_hhf_tac;
   262 
   263 fun gen_arule_tac tac j rules facts =
   264   EVERY' (gen_rule_tac tac rules facts :: replicate j Tactic.assume_tac);
   265 
   266 fun gen_some_rule_tac tac arg_rules ctxt facts = SUBGOAL (fn (goal, i) =>
   267   let
   268     val rules =
   269       if not (null arg_rules) then arg_rules
   270       else flat (ContextRules.find_rules false facts goal ctxt)
   271   in trace ctxt rules; tac rules facts i end);
   272 
   273 fun meth tac x = METHOD (HEADGOAL o tac x);
   274 fun meth' tac x y = METHOD (HEADGOAL o tac x y);
   275 
   276 in
   277 
   278 val rule_tac = gen_rule_tac Tactic.resolve_tac;
   279 val rule = meth rule_tac;
   280 val some_rule_tac = gen_some_rule_tac rule_tac;
   281 val some_rule = meth' some_rule_tac;
   282 
   283 val erule = meth' (gen_arule_tac Tactic.eresolve_tac);
   284 val drule = meth' (gen_arule_tac Tactic.dresolve_tac);
   285 val frule = meth' (gen_arule_tac Tactic.forward_tac);
   286 
   287 end;
   288 
   289 
   290 (* intros_tac -- pervasive search spanned by intro rules *)
   291 
   292 fun intros_tac intros facts =
   293   ALLGOALS (insert_tac facts THEN'
   294       REPEAT_ALL_NEW (resolve_tac intros))
   295     THEN Tactic.distinct_subgoals_tac;
   296 
   297 
   298 (* ML tactics *)
   299 
   300 structure TacticData = ProofDataFun
   301 (
   302   type T = thm list -> tactic;
   303   fun init _ = undefined;
   304 );
   305 
   306 val set_tactic = TacticData.put;
   307 
   308 fun ml_tactic (txt, pos) ctxt =
   309   let
   310     val ctxt' = ctxt |> Context.proof_map
   311       (ML_Context.expression pos
   312         "fun tactic (facts: thm list) : tactic"
   313         "Context.map_proof (Method.set_tactic tactic)" txt);
   314   in Context.setmp_thread_data (SOME (Context.Proof ctxt)) (TacticData.get ctxt') end;
   315 
   316 fun tactic txt ctxt = METHOD (ml_tactic txt ctxt);
   317 fun raw_tactic txt ctxt = RAW_METHOD (ml_tactic txt ctxt);
   318 
   319 
   320 
   321 (** method syntax **)
   322 
   323 (* method text *)
   324 
   325 type src = Args.src;
   326 
   327 datatype text =
   328   Basic of (Proof.context -> method) * Position.T |
   329   Source of src |
   330   Source_i of src |
   331   Then of text list |
   332   Orelse of text list |
   333   Try of text |
   334   Repeat1 of text |
   335   SelectGoals of int * text;
   336 
   337 fun primitive_text r = Basic (K (SIMPLE_METHOD (PRIMITIVE r)), Position.none);
   338 val succeed_text = Basic (K succeed, Position.none);
   339 val default_text = Source (Args.src (("default", []), Position.none));
   340 val this_text = Basic (K this, Position.none);
   341 val done_text = Basic (K (SIMPLE_METHOD all_tac), Position.none);
   342 fun sorry_text int = Basic (cheating int, Position.none);
   343 
   344 fun finish_text (NONE, immed) pos = Basic (close immed, pos)
   345   | finish_text (SOME txt, immed) pos = Then [txt, Basic (close immed, pos)];
   346 
   347 
   348 (* method definitions *)
   349 
   350 structure Methods = TheoryDataFun
   351 (
   352   type T = (((src -> Proof.context -> method) * string) * stamp) NameSpace.table;
   353   val empty = NameSpace.empty_table;
   354   val copy = I;
   355   val extend = I;
   356   fun merge _ tables : T = NameSpace.merge_tables (eq_snd (op =)) tables handle Symtab.DUP dup =>
   357     error ("Attempt to merge different versions of method " ^ quote dup);
   358 );
   359 
   360 fun print_methods thy =
   361   let
   362     val meths = Methods.get thy;
   363     fun prt_meth (name, ((_, comment), _)) = Pretty.block
   364       [Pretty.str (name ^ ":"), Pretty.brk 2, Pretty.str comment];
   365   in
   366     [Pretty.big_list "methods:" (map prt_meth (NameSpace.extern_table meths))]
   367     |> Pretty.chunks |> Pretty.writeln
   368   end;
   369 
   370 
   371 (* get methods *)
   372 
   373 val intern = NameSpace.intern o #1 o Methods.get;
   374 val defined = Symtab.defined o #2 o Methods.get;
   375 
   376 fun method_i thy =
   377   let
   378     val meths = #2 (Methods.get thy);
   379     fun meth src =
   380       let val ((name, _), pos) = Args.dest_src src in
   381         (case Symtab.lookup meths name of
   382           NONE => error ("Unknown proof method: " ^ quote name ^ Position.str_of pos)
   383         | SOME ((mth, _), _) => (Position.report (Markup.method name) pos; mth src))
   384       end;
   385   in meth end;
   386 
   387 fun method thy = method_i thy o Args.map_name (NameSpace.intern (#1 (Methods.get thy)));
   388 
   389 
   390 (* add method *)
   391 
   392 fun add_methods raw_meths thy =
   393   let
   394     val new_meths = raw_meths |> map (fn (name, f, comment) =>
   395       (Binding.name name, ((f, comment), stamp ())));
   396 
   397     fun add meths = fold (snd oo NameSpace.bind (Sign.naming_of thy)) new_meths meths
   398       handle Symtab.DUP dup => error ("Duplicate declaration of method " ^ quote dup);
   399   in Methods.map add thy end;
   400 
   401 val add_method = add_methods o Library.single;
   402 
   403 
   404 (* method_setup *)
   405 
   406 fun method_setup name (txt, pos) cmt =
   407   Context.theory_map (ML_Context.expression pos
   408     "val method: bstring * (Method.src -> Proof.context -> Proof.method) * string"
   409     "Context.map_theory (Method.add_method method)"
   410     ("(" ^ quote name ^ ", " ^ txt ^ ", " ^ quote cmt ^ ")"));
   411 
   412 
   413 
   414 (** concrete syntax **)
   415 
   416 structure P = OuterParse;
   417 
   418 
   419 (* basic *)
   420 
   421 fun syntax scan = Args.context_syntax "method" scan;
   422 
   423 fun simple_args scan f src ctxt : method =
   424   fst (syntax (Scan.lift (scan >> (fn x => f x ctxt))) src ctxt);
   425 
   426 fun ctxt_args (f: Proof.context -> method) src ctxt =
   427   fst (syntax (Scan.succeed (f ctxt)) src ctxt);
   428 
   429 fun no_args m = ctxt_args (K m);
   430 
   431 
   432 (* sections *)
   433 
   434 type modifier = (Proof.context -> Proof.context) * attribute;
   435 
   436 local
   437 
   438 fun thms ss = Scan.repeat (Scan.unless (Scan.lift (Scan.first ss)) Attrib.multi_thm) >> flat;
   439 fun app (f, att) (context, ths) = Library.foldl_map att (Context.map_proof f context, ths);
   440 
   441 fun section ss = Scan.depend (fn context => (Scan.first ss -- Scan.pass context (thms ss)) :|--
   442   (fn (m, ths) => Scan.succeed (app m (context, ths))));
   443 
   444 fun sectioned args ss = args -- Scan.repeat (section ss);
   445 
   446 in
   447 
   448 fun sectioned_args args ss f src ctxt =
   449   let val ((x, _), ctxt') = syntax (sectioned args ss) src ctxt
   450   in f x ctxt' end;
   451 
   452 fun bang_sectioned_args ss f = sectioned_args Args.bang_facts ss f;
   453 fun bang_sectioned_args' ss scan f =
   454   sectioned_args (Args.bang_facts -- scan >> swap) ss (uncurry f);
   455 fun only_sectioned_args ss f = sectioned_args (Scan.succeed ()) ss (fn () => f);
   456 
   457 fun thms_ctxt_args f = sectioned_args (thms []) [] f;
   458 fun thms_args f = thms_ctxt_args (K o f);
   459 fun thm_args f = thms_args (fn [thm] => f thm | _ => error "Single theorem expected");
   460 
   461 end;
   462 
   463 
   464 (* tactic syntax *)
   465 
   466 fun nat_thms_args f = uncurry f oo
   467   (fst oo syntax (Scan.lift (Scan.optional (Args.parens P.nat) 0) -- Attrib.thms));
   468 
   469 fun goal_args' args tac src ctxt = fst (syntax (Args.goal_spec HEADGOAL -- args >>
   470   (fn (quant, s) => SIMPLE_METHOD'' quant (tac s))) src ctxt);
   471 
   472 fun goal_args args tac = goal_args' (Scan.lift args) tac;
   473 
   474 fun goal_args_ctxt' args tac src ctxt =
   475   fst (syntax (Args.goal_spec HEADGOAL -- args >>
   476   (fn (quant, s) => SIMPLE_METHOD'' quant (tac ctxt s))) src ctxt);
   477 
   478 fun goal_args_ctxt args tac = goal_args_ctxt' (Scan.lift args) tac;
   479 
   480 
   481 (* outer parser *)
   482 
   483 fun is_symid_meth s =
   484   s <> "|" andalso s <> "?" andalso s <> "+" andalso OuterLex.ident_or_symbolic s;
   485 
   486 local
   487 
   488 fun meth4 x =
   489  (P.position (P.xname >> rpair []) >> (Source o Args.src) ||
   490   P.$$$ "(" |-- P.!!! (meth0 --| P.$$$ ")")) x
   491 and meth3 x =
   492  (meth4 --| P.$$$ "?" >> Try ||
   493   meth4 --| P.$$$ "+" >> Repeat1 ||
   494   meth4 -- (P.$$$ "[" |-- Scan.optional P.nat 1 --| P.$$$ "]") >> (SelectGoals o swap) ||
   495   meth4) x
   496 and meth2 x =
   497  (P.position (P.xname -- Args.parse1 is_symid_meth) >> (Source o Args.src) ||
   498   meth3) x
   499 and meth1 x = (P.enum1 "," meth2 >> (fn [m] => m | ms => Then ms)) x
   500 and meth0 x = (P.enum1 "|" meth1 >> (fn [m] => m | ms => Orelse ms)) x;
   501 
   502 in val parse = meth3 end;
   503 
   504 
   505 (* theory setup *)
   506 
   507 val _ = Context.>> (Context.map_theory
   508   (add_methods
   509    [("fail", no_args fail, "force failure"),
   510     ("succeed", no_args succeed, "succeed"),
   511     ("-", no_args insert_facts, "do nothing (insert current facts only)"),
   512     ("insert", thms_args insert, "insert theorems, ignoring facts (improper)"),
   513     ("intro", thms_args intro, "repeatedly apply introduction rules"),
   514     ("elim", thms_args elim, "repeatedly apply elimination rules"),
   515     ("unfold", thms_ctxt_args unfold_meth, "unfold definitions"),
   516     ("fold", thms_ctxt_args fold_meth, "fold definitions"),
   517     ("atomize", (atomize o fst) oo syntax (Args.mode "full"),
   518       "present local premises as object-level statements"),
   519     ("rule", thms_ctxt_args some_rule, "apply some intro/elim rule"),
   520     ("erule", nat_thms_args erule, "apply rule in elimination manner (improper)"),
   521     ("drule", nat_thms_args drule, "apply rule in destruct manner (improper)"),
   522     ("frule", nat_thms_args frule, "apply rule in forward manner (improper)"),
   523     ("this", no_args this, "apply current facts as rules"),
   524     ("fact", thms_ctxt_args fact, "composition by facts from context"),
   525     ("assumption", ctxt_args assumption, "proof by assumption, preferring facts"),
   526     ("rename_tac", goal_args (Scan.repeat1 Args.name) Tactic.rename_tac,
   527       "rename parameters of goal"),
   528     ("rotate_tac", goal_args (Scan.optional P.int 1) Tactic.rotate_tac,
   529       "rotate assumptions of goal"),
   530     ("tactic", simple_args (P.position Args.name) tactic, "ML tactic as proof method"),
   531     ("raw_tactic", simple_args (P.position Args.name) raw_tactic,
   532       "ML tactic as raw proof method")]));
   533 
   534 
   535 (*final declarations of this structure!*)
   536 val unfold = unfold_meth;
   537 val fold = fold_meth;
   538 
   539 end;
   540 
   541 structure BasicMethod: BASIC_METHOD = Method;
   542 open BasicMethod;